1. Field of the Invention
The present invention relates to a bubble sensor, and more particularly, concerns a sensor and method for detecting the presence of air bubbles in liquid, and more specifically the presence of air bubbles in liquid flowing through a tube.
2. Background Description
Bubble sensors are used in a wide variety of commercial, scientific, laboratory and medical instruments. For instance, in automated chemical analyzers useful for analyzing biological or chemical liquids, the amounts of liquid to be tested in the sample need to be controlled with some degree of precision. When analyzing a liquid sample for the concentration of a particular chemical or biological component thereof, the volume of the liquid sample is a factor that is used in the ultimate analysis. Also, when analyzing a large batch of samples, whether for pure analysis purposes or for statistical reasons, the volume of each sample being tested is often expected to remain constant from sample to sample.
Many automated chemical analysis instruments rely on transfer tubes or the like for pipetting samples into wells or containers for analysis. In the pipetting operation, and similar liquid handling procedures, liquid is drawn into the transfer tube by means of a pump or vacuum-assisted device. If the level of the source of liquid to be transferred is low, or due to other causes such as an air gap in the system, air is included in the liquid sample. When this occurs, the precision of the liquid volume to be tested is disturbed with the result that the liquid sample, with air or bubbles included therein, is susceptible to inaccuracies and errors in the testing procedures.
Bubble sensors are currently found in commercially available instruments, such as automated chemical analyzers. A typical bubble sensor includes a photodetector, such as a photodiode or phototransistor, diametrically opposed (180.degree.) to a light source such as a light emitting diode (LED) to sense air or bubbles in a tube through which liquid flows. Pin holes between the LED and tube, as well as between the tube and the photodiode, limit the detected light path between the LED and photodiode to a diameter smaller than the inside diameter of the tube. With the tube full of liquid, light is coupled from the LED across the diameter of the tube to the photodetector. When bubbles are present, the optical coupling is perhaps 25% to 50% less.
There is a need to improve the ability to detect air or bubbles in a tube for transporting liquid, for a number of reasons including the enhancement of the optical coupling efficiency. Such an improved bubble sensor would be most suitable for use in an automated analytical instrument such as described in copending and commonly assigned patent application Ser. No. 799,238, filed on Nov. 8, 1985, and entitled "Automatic Random Access Analyzer."